This invention relates to an improved electrodialytic water splitting process for materials that evolve gases of low aqueous solubility upon contact of the materials with aqueous acid. More particularly, this invention relates to the improved operation of two- and three-compartment electrodialytic water splitters by acidification, external to said water splitters, of a material comprising a salt of a first acid, capable of generating a gas upon acidification, with a hydrogen ion-enriched aqueous solution comprising a second acid generated and withdrawn from said water splitters.
Electrodialytic water splitting in bipolar membrane containing cells is a well known method of introducing hydrogen ions into one solution acidification while simultaneously introducing hydroxide ions into another solution (basification). Thus, electrodialytic water splitting of aqueous salts (e.g., NaCl) produces solutions of aqueous acid (e.g., HCl) and of aqueous base (e.g., NaOH). See, for example, U.S. Pat. No. 2,829,095 (Oda et al.) which discloses electrodialytic water splitting of aqueous salts such as sodium chloride and sodium acetate produce dilute aqueous acid and base solutions.
U.S. Pat. Nos. 4,082,835 and 4,107,015 (Chlanda et al.) disclose electrodialytic water splitting processes to recover SO.sub.2 from dilute gas streams containing same comprising the steps of (1) solution scrubbing said gas streams with an alkaline aqueous solution containing NaOH, Na.sub.2 SO.sub.3 and/or Na.sub.2 SO.sub.4 to produce an aqueous SO.sub.2 -containing salt solution comprising aqueous Na.sub.2 SO.sub.3 and NaHSO.sub.3 (2) regenerating alkaline aqueous scrubbing solution and producing sulfurous acid solution in a two-compartment electrodialytic water splitter; and (3) liberating SO.sub.2 gas. These processes suffer from a number of inherent disadvantages. For example, although the processes therefore may be effected in a two-compartment water-splitter, the composition of the solutions fed to both compartments is identical. Furthermore, the basic product solutions are mixtures, e.g., aqueous NaOH, Na.sub.2 SO.sub.3, and Na.sub.2 SO.sub.4 and, as such, cannot normally be used in other processes wherein relatively pure products are required. Moreover, the electrodialytic water splitters are operated under superatmospheric pressure to insure that aqueous sulfurous acid generated in the acid compartments of the water splitters does not evolve gaseous in order to increase SO.sub.2 the service lifetime of the water splitters beyond about four months. However, operating the water splitter under superatmospheric pressure results in higher cell voltage.
U.S. Pat. No. 4,238,305 (Gancy et al.) discloses a two-compartment electrodialytic water splitting process for converting relatively dilute aqueous solutions of sodium carbonate, sodium bicarbonate, trona or mixtures thereof into aqueous sodium hydroxide and carbon dioxide gas. However, the concentration of sodium carbonate in the solution which is fed to the acid compartment of the electrodialytic water splitter must be no more than 12 weight percent to prevent the precipitation of NaHCO.sub.3 from the solution. Furthermore, H.sub.2 CO.sub.3 is produced in the acid compartment and CO.sub.2 gas evolves therefrom. The sparingly soluble CO.sub.2 gas is detrimental to cell performance.
Several inorganic salts liberate gaseous products when they are acidified. These inorganic salts include sulfites (SO.sub.3.sup..dbd.) and bisulfites, (HSO.sub.3.sup.-), carbonates (CO.sub.3.sup..dbd.) and bicarbonates, (HCO.sub.3.sup.-) sulfides (S.sup..dbd.), hydrosulfides (HS.sup.-) and nitrites (NO.sub.2.sup.-). For example: EQU M.sup.+.sub.2 SO.sub.3.sup.-- +2H.sup.+ .fwdarw.M.sup.+ HSO.sub.3.sup.- +H.sup.+ M.sup.+ .fwdarw.H.sub.2 SO.sub.3 +2M.sup.+ EQU H.sub.2 SO.sub.3 .fwdarw.H.sub.2 O+SO.sub.2 .uparw. EQU M.sup.+.sub.2 CO.sub.3.sup.-- 2+H.sup.+ .fwdarw.M.sup.+ HCO.sub.3 +H.sup.+ -M.sup.+ .fwdarw.H.sub.2 CO.sub.3 +2M.sup.+ EQU H.sub.2 CO.sub.3 .fwdarw.H.sub.2 O+CO.sub.2 .uparw. EQU M.sup.+.sub.2 S.sup.-- +2H.sup.+ .fwdarw.M.sup.+ HS.sup.- +H.sup.+ M.sup.+ .fwdarw.H.sub.2 S.uparw.+2M.sup.+ EQU M.sup.+ NO.sub.2.sup.- +H.sup.+ .fwdarw.HNO.sub.2 +M.sup.+ .fwdarw.M.sup.++ 1/3HNO.sub.3 +1/3H.sub.2 O+2/3NO.uparw.
Huge quantities of waste streams contain carbonates and bicarbonates (for example, trona process liquors) and sulfite, bisulfite (for example, alkaline SO.sub.2 scrubbing solution), and these streams are amendable to recovering of valuable metal ion values, e.g., Na.sup.+, via electrodialytic water splitting. However, prior art electrodialytic water splitting processes disclose no method which allows recovery of these valuable metal ion values without the deleterious formation of bubbles of sparingly soluble gas in the acid compartments of the electrodialytic water splitters.